[32137-73-8]  · C10H13NSSi  · 2-Trimethylsilyl-1,3-benzothiazole  · (MW 207.40)

(silyl heterocycle that reacts with a variety of electrophiles)

Physical Data: bp 139 °C/15 mmHg; n25D = 1.57.

Solubility: sol all organic solvents.

Analysis of Reagent Purity: NMR.

Purification: vacuum distillation.

Handling, Storage, and Precautions: can be stored indefinitely in the refrigerator in the absence of moisture.

Reaction with Carbon Electrophiles.

The ability of this reagent to transfer the benzothiazol-2-yl unit to carbon electrophiles, is demonstrated by reactions with aldehydes, acyl halides, and chloroformates that give trimethylsilyl carbinols, ketones, and esters (eq 1).1

The reaction with benzaldehyde is performed under a dry nitrogen atmosphere at 160 °C for 40 h; acyl halides and Ethyl Chloroformate react under milder reaction conditions. The reaction of 2 equiv of 2-trimethylsilyl-1,3-benzothiazole and Phosgene2 occurs at 0 °C and provides a bis-heteroaryl ketone; Oxalyl Chloride2 is converted to the (benzothiazol-2-yl)-substituted diketone. In these reactions the use of the corresponding stannylated derivative leads in several cases to complex product mixtures. The reactivity toward electrophiles is increased in the presence of a catalytic amount of fluoride ion.3 The easy F--catalyzed desilylation, promoted by Cesium Fluoride or SiO2-supported Tetra-n-butylammonium Fluoride, allows reaction with a wider range of electrophiles such as saturated lactones (eq 2). d-Valero- and ε-caprolactone react smoothly under F- catalysis, affording the corresponding keto alcohols from acyl-oxygen bond cleavage. Ring size effects are also seen; reaction of b-propiolactone fails because of ring opening and polymerization, whereas g-butyrolactone gives only modest conversion yields (15%). With cyclohexen-2-one as an electrophile, conjugated addition of the benzothiazol-2-yl carbanion occurs, with formation of the Michael adduct.

Stereoselective Additions.

The addition of 2-trimethylsilyl-1,3-benzothiazole to a-asymmetric aldehydes proceeds4 under mild conditions in good chemical yield and with significant stereoselectivity. In particular, the reaction with D-glyceraldehyde gives the anti-isomer with 80% diastereomeric purity (eq 3). The diastereoselectivity appears to vary depending on the azole nucleophile. Reaction of 2-lithiobenzothiazole with the same aldehyde (Et2O, -78 °C) shows a complete lack of stereoselectivity. Unmasking the formyl group, using suitable procedures from the literature,5a,b allows 2-trimethylsilyl-1,3-benzothiazole to be used as a formyl anion equivalent in the diastereoselective synthesis of a-hydroxy carbonyl derivatives.

Synthesis of Heterocyclic-Substituted Tertiary Phosphines.

Electrophilic cleavage of the C-Si bond of 2-trimethylsilyl-1,3-benzothiazole by Phosphorus(III) Chloride, PhPCl2, and Ph2PCl, affords the tris-, bis-, and mono(benzothiazol-2-yl)phosphines in good yields (eq 4). The procedure involves the electrophilic cleavage of the C-Si bond and formation of a C-P bond.6 The reactions are conducted neat and the phosphorus derivatives appear quite air-stable even at room temperature. Attempts to prepare tris(benzothiazol-2-yl)phosphines by reaction of the corresponding heteroaryl organolithium reagent and PCl3 were unsuccessful.

1. Pinkerton, F. H.; Thames, S. F. JHC 1971, 8, 257.
2. Jutzi, P.; Gilge, U. JHC 1983, 20, 1011.
3. Ricci, A.; Fiorenza, M.; Grifagni, M. A.; Bartolini, G. TL 1982, 23, 5079.
4. Dondoni, A.; Fogagnolo, M.; Medici, A.; Pedrini, P. TL 1985, 26, 5477.
5. (a) Altman, L. J.; Richheimer, S. L. TL 1971, 4709. (b) Chikasita, H.; Ishibaba, M.; Ori, K.; Itoh, K. BCJ 1988, 61, 3637.
6. Moore, S. S.; Whitesides, G. M. JOC 1982, 47, 1489.

Alfredo Ricci

Università di Bologna, Italy

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